1. Field of the Invention
The present invention relates to an organic electroluminescence device.
2. Description of the Related Art
Organic electroluminescence devices (hereinafter referred to as “organic EL devices”) are each an electron device obtained by interposing a thin film containing a fluorescent or phosphorescent organic compound between paired electrodes. An electron and a hole are injected from the respective electrodes. As a result, an exciton of the fluorescent or phosphorescent compound is produced. Each of the organic EL devices emits light when the exciton returns to its ground state. The organic EL devices have recently shown significant progress, and each have such characteristics as described below. Each of the devices shows high luminance at a low applied voltage, has a wide variety of luminous wavelengths, responds at a high speed, and enables the thinning and weight reduction of a light emitting device. Those characteristics suggest the potential of the devices to find use in assorted applications. In addition, research and development for improving the performance of each of the organic EL devices have been vigorously advanced in recent years.
The organic EL devices are each an electron device having an organic compound layer between paired electrodes. Here, the organic compound layer is a single layer or a laminate of multiple layers having at least an emission layer and obtained by appropriately inserting a charge transport layer (a hole transport layer or an electron transport layer) or a charge injection layer (hole injection layer or electron injection layer) between an electrode and the emission layer depending on purposes. In addition, the emission layer in the organic compound layer is not necessarily needed to be a single layer. Japanese Patent No. 3,949,214 discloses an organic EL device whose emission layer is formed of two layers for improving its light emitting efficiency and durability.
However, the organic EL device disclosed in Japanese Patent No. 3,949,214 had insufficient light emitting efficiency and insufficient durability. In addition, the organic EL device disclosed in Japanese Patent No. 3,949,214 is of a constitution in which: the ionization potential of a host increases from an anode to an emission layer on an anode side; and the electron affinity of a host reduces from a cathode to an emission layer on a cathode side. Therefore, hole and electron barriers arise at an interface between the host in the emission layer on the anode side and the host in the emission layer on the cathode side. As a result, charge accumulates at the interface, and quenching may occur owing to the charge. Further, a concern has been raised about the escape of a hole toward an electron transport layer in the organic EL device disclosed in Japanese Patent No. 3,949,214 because a hole barrier between the emission layer on the cathode side and the electron transport layer is small.